1. Field of the Invention
The invention relates to steam generators for nuclear power plants and, more particularly, to a method of assessing the servicing needs of such a steam generator.
2. Background
A nuclear steam generator comprises a vertically oriented shell, a plurality of U-shaped tubes disposed in the shell so as to form a tube bundle, a tube sheet for supporting the tubes at the ends opposite the U-like curvature, a dividing plate that cooperates with the tube sheet and a hemispheric channel head to form a primary fluid inlet header at one end of the tube bundle and a primary fluid outlet header at the other end of the tube bundle. A primary fluid inlet nozzle is in fluid communication with the primary fluid inlet header and a primary fluid outlet nozzle is in fluid communication with the primary fluid outlet header. The steam generator secondary side comprises a wrapper disposed between the tube bundle and the shell to form an annular chamber made up of the shell on the outside of the wrapper on the inside, and a feedwater ring disposed above the U-like curvature end of the tube bundle.
The primary fluid having been heated by circulation through the reactor core enters the steam generator through the primary fluid inlet nozzle. From the primary fluid inlet nozzle, the primary fluid is conducted through the primary fluid inlet header, through the U-tube bundle, out the primary fluid outlet header, through the primary fluid outlet nozzle to the remainder of the reactor system. At the same time, feedwater is introduced to the steam generator secondary side through a feedwater nozzle which is connected to the feedwater ring inside the steam generator. Upon entering the steam generator, the feedwater mixes with water returning from moisture separators positioned above the U-tube bundle, referred to as the recirculation stream. This mixture, called the downcomer flow, is conducted down the annular chamber adjacent to the shell between the shell and the wrapper until the tube sheet near the bottom of the annular chamber causes the water to reverse direction, passing in heat transfer relationship with the outside of the U-tubes and up through the inside of the wrapper. While the water is circulating in heat transfer relationship with the tube bundle, heat is transferred from the primary fluid in the tubes to the water surrounding the tubes, causing a portion of the water to be converted to steam. The steam then rises and is conducted through a number of moisture separators that separate any entrained water from the steam, and the steam vapor then exits the steam generator and is circulated through typical electrical generating equipment to generate electricity in a manner well-known in the art.
Loose parts may enter the steam generator through the feedwater stream and can cause damage to the heat transfer tubes in the tube bundle. This damage can results in having to plug or repair the damaged tubes to avoid contamination of the secondary fluid. In extreme cases, the damage can lead to a tube leak and forced outage with significant expense to the plant. Therefore, it is important to prevent foreign objects from entering the steam generator and/or to remove the loose parts from the steam generator before damage occurs. Co-pending application Ser. No. 11/563,742, filing date Nov. 28, 2006, describes one means of trapping the loose parts so that they do not enter the tube bundle. However, periodic maintenance is still required to remove the loose parts from the trapping mechanism before it becomes ineffective.
In addition, the tube bundle has as number of parallel support plates that are arranged in tandem and spaced along the longitudinal length of the bundle, through which the heat exchange tubes pass and are supported against vibration. The contact area between the tubes and the tube support plates tend to be hot with respect to the surrounding environment. The secondary water circulating in the steam generator tends to dissipate this heat if it is permitted to flow directly against the contact areas. However, fine particles of magnetite formed at relatively high temperatures within the circulating secondary water tend to collect and build up sludge patches about the tube openings, particularly the contact areas, thus preventing the secondary water direct access to the contact areas and the dissipation of heat therefrom. As the sludge patches build up, non-volatile impurity accumulation occurring at the contact areas is not washed away by the circulating secondary water, thus leading to dry-out and corrosion of the contact areas. It is desirable periodically to decrease the sludge patches to minimize this corrosive effect. In addition, due to the change of phase of the liquid on the secondary side form water to steam, tube sheet scale builds up around the tubes and forms a collar which can similarly result in corrosion. Furthermore, the change in phase results in a sludge that reduces the efficiency of the generator. Therefore, it is highly desirable to service the generators at periodic intervals to reduce the deleterious effects of the foregoing foreign matter that collects on the secondary side.
Unless there is a significant break in the stream generator tubes, the steam generators are typically serviced when the plant is shut down for other reasons that absolutely necessitate shuttling down the system, because of the expense of replacement power. Typically, outages occur at the end of the refueling cycles. However, even then, it may not be necessary to bear the expense of servicing any or all of the steam generators at each refueling outage if a system could be developed for assessing the performance of the steam generator.
Accordingly it is the object of this invention to develop a method of assessing the performance of a steam generator that will enable a plant operator to determine when and what kind of service is required based upon the operating expectations of the plant.